Most car batteries
are lead/acid type
- which means that they contain a series of lead plates and filled with
sulfuric acid. 35% Sulfuric Acid is very dangerous and should only be
handled by acid knowledgeable professionals. The following battery information
is for informational purposes only. We would not sell this information
nor would we suggest that anyone use this due to the risk/reward involved.

There are
various types of car batteries -

The sealed or (MF)
maintenance free type has no openings, one way vents release internal
gas pressure buildup, so there is no way to add an additive

Older style automotive
batteries have a series of screw caps, one for each cell where an additive
can be added

Many modern style
batteries have a press on cap that covers a number of cells that can
be pried off with a tool to access the acid in the cells and add an
additive.

The 2 main
additives used to extend battery life are epsom salts
and EDTA. You can get epsom salts at the drug store and there
are companies selling EDTA in their battery life extending products. To
make an additive from epsom salts you dissolve it in hot distilled water
which is then added to the acid in the battery. The idea is
to remove the sulfate buildup on the plates, so the battery can fully
charge again.

Using these
additives could get you a small amount of extra
life from your battery, but that's about it according to wikipedia.

Consider the
related problems -

Suppose you get
an extra 2 weeks out of the battery and your car gets stuck, who knows
where, & it costs you more than a new battery for towing. You need
a new one anyway.

While someone is
adding the additive to the battery, the very strong sulfuric acid gets
on his skin or in the eyes and the trip to the doctor costs you more
than a new battery.

The additive does
not work and you wasted your time and money when the best you can expect
is a short extended life

Other associated
risks are explosion from hydrogen gas from the battery, breathing acid
fumes, and acid burning things it gets on

Remember,
this is supplied for informational purposes only and
the author feels strongly against trying to extend a lead/acid batteries'
life with additives to the acid.

There are many companies
that sell electronic devices to de-sulfate batteries. This is the recommended
solution for a long lasting battery, but of course there is a price.